Photosensitive or photoreactive polymers are commonly used in coatings which have properties that can change when exposed to light. Such a change in properties is usually a change in solubility and results in a sharp distinction between exposed and unexposed areas. Photocross-linking and photoinitiated polymerization are commonly used to exploit the photosensitivity of materials. This invention relates to photocross-linking between ethylenically unsaturated sites. Exposure to light of the photosensitive or photoreactive material results in a solubility change and creation of image in the resist material. Most commonly, images are produced by solvent development, either negative or positive modes. Photoresist polymer compositions must fill a number of physical property dependent requirements under practical working conditions. The most important, and most difficult, requirements to fulfill is image discrimination, thermal stability, ink resistance, and etch resistance when exposed. For image discrimination, the developing solvent must remove the soluble portion of the exposed imagery without distorting or swelling the insoluble areas to give a sharp, true image. In the past, resist polymers were required only to withstand liquid aqueous etchants, for example, ammonia fluoride, hydrofluoric acid, etc. However, present trends towards other etching modes require polymers with a variety of properties.
Most commonly, photomodification of polymer functionality depends on the modification of the solubility of polymer-bound chromophore units upon light absorption. A change in functionality modifies the solubility of the polymer through pendant groups in selected solvents. For example, with sufficient exposure to light, a polymer having diazo ketone units becomes hydrophilic. A variety of photo-chemical systems have been used in photoresist applications including cinnamates, chalcones, p-azidophenyls, azidophthalates, p-phenylene, bis(acrylates), etc. One class of compounds used in forming photoinitiated dimerization is set forth in references including K. Ichimura, J. Poly. Sci. 20, 1411, 1982; K. Ichimura, et al., J. Poly. Sci. 20, 1419, 1982; Ichimura, et al., U.S. Pat. No. 4,777,114 (1988); U.S. Pat. No. 4,564,580 (1986); European Patent No. 130 804 (1984); Japanese Patent No. 63/198045 (1988); Balfour, European Patent No. 313 221 (1989); Sperry, European Patent No. 092 901 (1983); etc.
This technology involves using an aldol-type condensation reaction to graft a photodimerizable group onto a polyvinyl alcohol backbone as shown in a cross-linked and uncross-linked form. ##STR1## Unfortunately, the Ichimura, et al. technology suffers from the following drawbacks: (1) the materials remain water sensitive at any pH due to residual unreacted polyvinyl alcohol functionality and (2) the materials are expensive.
Other water sensitive or water soluble polymeric materials are disclosed in Mukunoki et al., U.S. Pat. No. 4,917,993, which discloses the use of a particular class of water soluble synthetic or natural polymers in silver halide photographic materials. The synthetic polymers disclosed in Mukunoki preferably have nonionic and/or anionic groups appended onto a poly(methyl) backbone which may or may not be otherwise substituted. These appended groups are linked to the poly(methyl) backbone through the following oxygen-containing linking groups: --CONH--, --NHCO--, --COO--, --OCO--, or --O--.
In order to satisfy the increasing demands for more versatile photoresist materials, we have sought to develop photoreactive or photosensitive polymers that satisfy demands in the art. Useful screen printing inks must be adherent to the substrate, uniform in drying, and exhibit thixotropic flow. In systems which are water-resistant, more specifically, water-based ink resistant, the stencil must exhibit controlled water solubility and cross-linkability. In other words, the materials should have properties that aid in controlling the water solubility of the polymer system. The materials, under use conditions, should be water soluble when unexposed and water resistant when exposed. The materials, after exposure, which must be removed, must be rendered sensitive to water, i.e., capable of selective or controlled dispersion or dissolution in order to remove or reclaim the photosensitive stencil or photoresist. We have found that pH sensitivity of the photoresist can be used as a primary tool for attaining the above listed goals. We have found that basic (pH&gt;7 or alkaline reacting) photo-polymers enable us to achieve the goals set forth above. Using acid solubilization, the unexposed photopolymers are inherently water soluble and are easily formulated and used in aqueous solutions. However, in alkaline or basic aqueous solutions, the materials can be quite insoluble resulting in a waterresistant stencil formed on a substrate. After adequate cross-linking, the formed image, either positive or negative, has substantial solvent resistance (aqueous and organic) in both acid and basic solution.
Therefore, a polymer having pendant basic insoluble and acid soluble photocross-linking groups in a photosensitive system would satisfy the requirements and constraints set forth above for photosensitive stencil products that can be formulated in a liquid or a pre-cast film for positive and negative imaging.